This invention relates generally to time delay systems, and, more particularly, to an optical time delay system for electrical signals which converts these electrical signals into optical signals in order to provide a mode dependent time delay.
There are numerous electronic devices and components in which it is desirable to utilize an electronic signal which has been delayed in time by a prescribed and controllable amount. A controlling signal is used to select the delay. Heretofore the time delaying of electrical signals has been accomplished by several methods including (1) the switching-in of different lengths of coaxial cable in the manner described in U.S. Pat. No. 3,781,722; and (2) the operation of electronic circuit components such as integrated circuits, discrete transistors, and charge-coupled devices.
Some of the more commonly referred to delay circuits include: (1) the multivibrator delay circuit in which a cathode-coupled or emitter-coupled monostable multivibrator may be used as an approximately linear delay circuit; (2) a linear time delay circuit which makes use of a linear saw tooth generator, such as the boot strap or Miller integrator, whose output is compared with a calibrated DC reference voltage level; (3) a circuit that combines the functions of a gate waveform generator, a clamp, and a linear saw tooth generator; and (4) a circuit that combines the Miller integrator saw tooth generator with the gating function and wherein the output is applied to a comparator in a complete linear time delay circuit.
Unfortunately the deficiencies of the prior art electronic delay circuits are numerous. For example, the coaxial devices are bulky and suffer from attenuation and distortion at high frequencies. The electronic devices have cost and complexity factors which markedly increase as the signal bandwidth and/or frequency go up to 1 GHz and beyond. In fact, in some cases the signal amplitude may be adversely affected, and equalization systems or circuits may also be required. The delays in such electronic circuits are many times selected with a potentiometer, rather than being under computer control. In the charge-coupled device approach, very complicated clocking networks are required which cause a considerable drawback. In the SAW devices it is extremely difficult to alter the delay factor. Generally in all such electronic time delay circuits, it is difficult to obtain ultra short delays in the 0.05 ns range. Furthermore, without using free-space propagation, it is difficult to provide remote transmission of the delay signal.
Consequently, and as is clearly evident by the above analysis of such electronic time delay devices, the prior art electronic devices do not satisfy the simultaneous requirements of compactness, simplicity, remote "invulnerable" transmission, multi-gigahertz bandwidth, constant amplitude, cost effectiveness, ultra short delays, numerous delay steps, low-power computer control and rapid updating of delay that is generally required in some of the newer "optical/microwave" systems.
One such new application of electrical time delay circuits can be found in the optical/microwave phased-array antenna. In such a hybrid antenna there are stringent requirements of computer-control, viable coupling and, of course, variable delay devices for steering the radiation beam. Furthermore, rapid changes in beam pointing direction are desirable, and such a factor imposes a rapid transition time on the programmable devices incorporated therein. In fact, transition times as short as 1 ns are desirable, although 0.01 to 10 microseconds would be acceptable in some cases. Still further, there are instances in which bandwidth of at least 2 GHz is needed, together with discrete accurate control of the time delay. Even further, approximately 100 equal steps of delay per device are frequently required, with the minimum step being about 0.05 ns. In conjunction with all of the above requirements it is generally conceded that the control power should be less than 0.05 watts. As is clearly evident from the above explanation of the prior art electronic devices, such purely electronic time delay circuits fall short in requirements for the new "optical/microwave" systems in use today and in the future.
It has been recognized in the prior art, as exemplified by U.S. Pat. No. 3,777,149 issued to Marcatili and U.S. Pat. No. 4,050,782 issued to Uchida et al, that an optical input into a fiber over the entire angular range of the fiber by a wide-angle optical source produces an optical output which is decomposed into angular ranges or "packets." Both of the above patents are directed toward equalizing the inherent differences in modal delay time of the fiber by adding unequal lengths of fiber to the various focused modes (Uchida et al), or by adding unequal delay networks (Marcatili) to those separated detected modes. In both of these patents, however, there is no desire or system disclosed to produce unequal time delays in an incoming electrical signal. Consequently, it is readily apparent that a need still arises for an effective and reliable optical time delay system for electrical signals.